Silicon based structural ceramics such as CMCs, monolithic SiC and Si3N4 require protective coatings to prevent recession in water vapor rich combustion environments encountered in advanced gas turbines. In order to enhance the performance and life of such components, it is essential to use protective coatings known as environmental barrier coatings (EBCs). While a large number of patents and patent applications describe EBC compositions, there is a relative scarcity of methods directed to applying such protective coatings to complex shaped parts. Various industries employ complex shaped parts. One such industry is aircraft manufacturers and their gas turbine engine components having complex part shapes such as integral vane rings and integrally bladed rotors.
Suitable coating processes for such complex shaped parts must provide thick, dense coatings at a low cost and rapid production rate. Both plasma spraying and physical vapor deposition processes are line of sight processes that are not practical for rapidly coating complex geometries. A non-line of sight process often used to provide dense coatings is chemical vapor deposition (“CVD”). Although this technique provides thick, dense coatings, CVD processes are expensive, slow and require a great deal of process development and operator skill.
Recently, a coating process involving electrophoretic deposition (“EPD”) as a non-line of sight method was disclosed U.S. patent application Ser. No. 10/914,925, filed on Aug. 9, 2004 and assigned to the assignee of reference in the present application, United Technologies Corporation. However, EPD includes certain manufacturing drawbacks which diminish the process' commercial appeal. EPD processes require an electrically conductive substrate and a complex electrode design to deposit uniform coating(s) upon the substrates.
Another coating process involves sol-gel. Sol-gel processes are often used to coat substrates possessing complex geometries. Sol-gel processes produce dense coatings in a rapid and inexpensive manner. However, sol-gel processes produce dense coatings having a thickness of only a few microns, which is makes the process unsuitable where the coating must be thick and dense enough to withstand exposure to harsh environmental conditions.
Consequently, there exists a need for a cost effective process for preparing and applying dense environmental barrier coatings effective at limiting the diffusion of water vapor and providing extended service life of complex shaped parts in all applicable industries.